Headers parsing performance optimization

[bogdan]

Among other tiny changes this PR contains the following optimization:

-        selected = select_field_for(field.name)
-
-        if selected.any? && limited_field?(field.name) 
+        if limited_field?(field.name) && (selected = select_field_for(field.name)) && selected.any?

This allows select_field_for(that is pretty slow when number of headers is high) be called only when limited_field? returns true.

About 30% performance improvement for Mail::Header.new method when there are a lot of unknown headers.

[mikel]

Nice, thanks :)